WO2006008541A2

WO2006008541A2 - Electronic apparatus

Info

Publication number: WO2006008541A2
Application number: PCT/GB2005/002878
Authority: WO
Inventors: Rodney Miller
Original assignee: Stenzel Security Limited
Priority date: 2004-07-23
Filing date: 2005-07-22
Publication date: 2006-01-26
Also published as: WO2006008541A3; EP1820257A2; CN101023574A; GB0416519D0; JP2008507811A

Abstract

The invention provides a device for producing and focussing an electron beam; the device comprising an evacuated chamber (4), a source of electrons (2), a target (6), means for creating an electric field to accelerate a stream of the electrons constituting the electron beam through the chamber towards the target, and means (12) for providing a magnetic field crossing the electron beam; characterised in that the magnetic flux of the magnetic field progressively reduces in the direction of the target.

Description

ELECTRONIC APPARATUS

This invention relates to a device for producing and focussing an electron beam and to a method of increasing the energy of an electron beam.

Background of the Invention

There are many types of electronic equipment that make use of electron beams and examples include cathode ray tubes, particle accelerators and X-ray machines.

Electron beam devices typically comprise a source of electrons, means for producing an electric field to accelerate the electrons and a target for the electron beam. The source of electrons, often referred to as an "electron gun", usually takes the form of a cathode element which is heated, either directly by means of a current passing through the cathode or by indirect heating, causing electrons to be emitted from the cathode. The electrons are accelerated by an anode and then focussed to create the beam which then passes through an evacuated chamber and is incident upon a target such as a screen or a target electrode.

Summary of the Invention

The invention provides a method of increasing the energy of a stream of electrons incident upon a target by subjecting the electrons to a magnetic field of varying, i.e. reducing, magnitude as the electrons move along a predefined path to the target.

By means of the method of the invention, the electron beam voltage detected by a target can be substantially increased.

In a first aspect, the invention provides a device for producing and focussing an electron beam; the device comprising an evacuated chamber, a source of electrons, a target, means for creating an electric field to accelerate a stream of the electrons constituting the electron beam through the evacuated chamber towards the target, and means for providing a magnetic field crossing the electron beam; characterised in that the flux of the magnetic field progressively reduces in the direction of the target. In another aspect, the invention provides a device for producing and enhancing the energy of an electron beam; the device comprising an evacuated chamber, a source of electrons, a target, means for creating an electric field to accelerate a stream of the electrons constituting the electron beam through the evacuated chamber towards the target, and means for providing a magnetic field crossing the electron beam; characterised in that the flux of the magnetic field progressively reduces in the direction of the target and whereby the energy of the electron beam is enhanced as it passes though the magnetic field.

In a further aspect, the invention provides a device for producing and increasing the voltage of an electron beam; the device comprising an evacuated chamber, a source of electrons, a target, means for creating an electric field to accelerate a stream of the electrons constituting the electron beam through the evacuated chamber towards the target, and means for providing a magnetic field crossing the electron beam; characterised in that the flux of the magnetic field progressively reduces in the direction of the target, and whereby the voltage of the electron beam is increased as it passes though the magnetic field.

In a still further aspect, the invention provides a power supply device; the power supply device comprising an evacuated chamber, means for producing a stream of electrons, a target connected to an output so that power can be drawn off the target, means for creating an electric field to accelerate the stream of electrons through the evacuated chamber towards the target, and means for providing a magnetic field crossing the electron beam; characterised in that the flux of the magnetic field progressively reduces in the direction of the target.

The magnetic field typically is substantially perpendicular to the electron beam.

The flux of the magnetic field can reduce in a continuous manner or it can reduce in a series of stepped reductions. It is preferred however that the flux of the magnetic field reduces in a continuous manner. The rate of change in the flux of the magnetic field with distance towards the target is typically substantially constant, i.e. the flux typically reduces in a rectilinear manner towards the target.

Alternatively, the rate of change in the flux of the magnetic field may vary in an increasing or decreasing manner in the direction of the target. Thus, in this case, for example, the magnetic flux may reduce in a curvilinear manner towards the target.

The means for providing the magnetic field may take the form of an array of two or more (e.g. two, four, six or eight) shaped magnets arranged about the stream of electrons, the shape of the magnets determining the reduction in magnetic flux in the direction of the target.

The array of magnets can comprise at least one pair (e.g. one, two or three pairs) of opposed magnets arranged about the stream of electrons, i.e. one magnet either side of the stream of electrons.

Preferably there are at least two pairs of opposed magnets arranged about the stream of electrons.

The magnets can take the form of elongate strips that taper in the direction of the target. The strips are typically of uniform thickness along their length. The angle of the taper will therefore determine the magnitude of the reduction in magnetic flux provided by the magnets.

The magnets may be trapezoidal in plan and have a wider end and a narrower end, the narrower end being nearer the target. Magnets of his shape are typically used when the evacuated chamber is linear in form and wherein the source of electrons and the target are at opposing ends of the linear chamber.

Alternatively, the shaped magnets may be spiral shaped in plan and have a wider end and a narrower end, the narrow end being at the innermost point of the spiral. Magnets having this shape may be used in devices where the evacuated chamber is an annular chamber which surrounds the target, rather than a linear chamber with a target at one end. In each case, typically, the magnetic field strength provided by the magnets is at least 0.001 Tesla, more usually at least 0.01 Tesla, preferably at least 0.05 Tesla and most preferably at least 0.1 Tesla.

The magnets are preferably ceramic magnets.

The evacuated chamber can be a permanently sealed chamber (e.g. a sealed ceramic tube, or any other high vacuum/ultra high vacuum sustaining material chamber), for example as in a television or computer monitor cathode ray tube, or can be connected to a vacuum pump. The evacuated chamber can be provided with an isolation valve for connection to a vacuum pump, the isolation valve enabling the chamber to be evacuated to the required pressure using the vacuum pump, closure of the isolation valve serving to seal the chamber to preserve the vacuum therein and isolate the chamber from the vacuum pump.

The evacuated chamber is typically evacuated to a pressure of 1 x 10^"7 Torr or less, more typically 5 x 10^'8 Torr or less and most preferably 1 x 10^"8 Torr or less.

A "getter" may be used to enhance or maintain the vacuum or to compensate for leakage into the evacuated chamber. Getters are well known and can be either physical or chemical getters. Physical getters include materials such as zeolites that function by adsorbing gases. Chemical getters are typically metals that function by reacting with gases such as oxygen, carbon dioxide, nitrogen, carbon monoxide and water vapour to form a low vapour pressure solid. Getters can be, for example, coated onto the inner surface of the evacuated chamber.

The getter can take the form of a getter pump and such pumps are well known.

The source of electrons can be an electron gun of conventional type comprising a cathode formed from or coated with an electron emitting material (e.g. a metal oxide coated cathode) that emits electrons when heated by a current passing through the cathode or a heating element adjacent the cathode, together with one or more focussing anodes to accelerate the electrons and collimate the electrons into a beam. The metal oxide coated cathode can be, for example a barium oxide cathode. Alternatively, a lanthanum hexaboride (LaB₆) cathode may be used. Lanthanum hexaboride cathodes are widely available and can be obtained from inter alia Cathay Advanced Materials Limited, Guangdong, China, or Kimball Physics Inc., Wilton, New Hampshire, USA).

The device of the invention can be any device that makes use of an electron beam and examples include cathode ray tube devices such as television and computer monitors, particle accelerators, x-ray machines, power electronic systems and un-interruptible power supplies.

Alternatively, or additionally, the device of the invention can be used as a power supply, the target being connected to an output so that current/power can be drawn off the target. Preferably, a converter, which converts the DC current from the target into AC current, is interposed between the target and the output. Electrical (preferably AC) output from the device may be used to power a range of electronic and electrical devices. In one embodiment, a portion of the electrical output from the device may be used to generate the electron beam and/or used to power a getter pump connected to the evacuated chamber.

The device of the invention can be used as a portable or stand alone power supply that does not require connection to a source of mains electricity. When used in this way, a battery may be included to provide sufficient power to initiate the generation of the electron beam.

Brief Description of the Drawings

Figure 1 is a schematic view of a device according to one embodiment of the invention.

Figure 2a illustrates a shaped magnet forming part of the device of Figure 1.

Figure 2b is a plan view of the magnet shown in Figure 2a. Figure 3 is a schematic illustration of the motion of an electron through the device of Figure 1.

Figure 4 is a schematic view illustrating the forces acting on an electron as it passes along the device of Figure 1.

Figure 5 is a diagrammatic illustration demonstrating how the geometry of the shaped magnets can be empirically based on an ideal spiral.

Figure 6 is a schematic view of the device as shown in Figure 1 but incorporating a power feedback loop.

Figure 7 is a block diagram of the components of the DC/ AC converter used in the device of Figures 1 to 6.

Figure 8 is a side elevation of the device shown in Figures 1 and 6.

Detailed Description of the Invention

The invention will now be illustrated, but not limited, by reference to the specific embodiment shown in Figures 1 to 8.

The device shown in Figure 1 comprises an electron gun 2 mounted at one end of evacuated tube 4, with a target electrode 6 being mounted at the other end of the evacuated tube. The electron gun can be of conventional type, for example of the type typically used in a cathode ray tube device such as a television or a computer monitor. Thus, the gun has a metal oxide coated emitter cathode 8 of diameter D that emits electrons when heated by heating filament 9. The electrons emitted by the cathode element are accelerated by the annular accelerating anode 10 which may have a potential of, for example, IkV.

At the far end of the evacuated chamber is located a target 6 which can be connected to a load or can form part of a screen. When the target is a screen, further deflecting or focussing arrangements (not shown) may be used to direct the beam onto particular regions of the screen in known fashion. Arranged about the evacuated tube 4 are two opposed pairs of shaped magnets 12 of which one pair is shown in Figure 1. In this embodiment, the magnets are permanent magnets although electromagnets may be used in addition to or instead of the permanent magnets.

As shown in Figures 2a and 2b, the magnets are generally flat and of uniform thickness along their length. In plan, the magnets are of trapezoidal form and taper in the direction of the target. The angle of taper θ can vary but preferably it is empirically based on an ideal spiral (also known as a logarithmic spiral), straightened out, the dimensions being dependent on the diameter of the emitter cathode and the diameter of the target. The manner in which the shape of the magnets can be determined is illustrated by Figure 5.

Figure 5 shows a pair of curves S¹ and S² that each follow an ideal spiral (logarithmic spiral) path to the centre of the spiral. As is known from classic mathematical theory, in an ideal spiral, the successive radii (e.g. r¹, r² and r³) at 90° intervals are in golden section (φ) proportion to each other. The distance W¹ between the two spiral lines at the outer end of the spiral corresponds to the width W¹ of the larger end of the magnet shown in Figure 2. This width is selected according to the diameter D of the emitter cathode 8 and is typically between 2.00 and 2.75 times (and more usually 2.25 and 2.75 times) the diameter D of the emitter cathode. Preferably the width W¹ is approximately 2.5 times the diameter D of the emitter cathode 8. The distance W nearer the centre of the spiral corresponds to the width W² of the narrower end of the magnet shown in Figure 2. The width W² is selected according to the diameter of the target electrode. With W¹ and W² set according to the dimensions of the emitter electrode and target electrode respectively, the length of the magnet is then set by the length of the spiral path between the two mid-points P¹ and P². Thus, the shape of the magnet can be viewed as being a straightened out section of an ideal spiral.

The shaped magnets 12 create a magnetic field B perpendicular to the electrical field E moving the electrons towards the target 6. As a consequence of the tapering shape of the magnets, the magnetic flux reduces along the length of the magnet in the direction of the target, since the flux per unit area changes.

Electrons passing through the evacuated tube are therefore subjected to a changing magnetic flux as they move along the tube. The forces on the electrons at various points along the tube are shown schematically in Figure 4, and the result of the combination of the electrical and magnetic fields is that the electrons exhibit a spiralling motion along the tube from the electron gun 2 in the direction of the target 6 as shown in Figure 3.

The arrangement shown in the Figures has the effect of accelerating the electrons in the electron beam thereby resulting in the production of a higher beam voltage at the target.

Comparative tests were carried out between a device as shown in the Figures and a similar device in which parallel straight sided magnets were used to give a constant magnetic flux along the tube. In the test device, the distance from electron source to target was 460 mm, and the dimensions of the trapezoidal magnets were as follows: length - 438 mm, thickness - 7 mm, base width of trapezoid 35 - mm, narrower end width - 4 mm. The following results were obtained.

Each magnet, both straight sided and tapering trapezoidal, had a field strength, B, of 0.1 Tesla. However, whereas the flux Φ was constant for the straight section, the flux varied with the trapezoidal magnet due to the changing area of the magnets. The comparative tests showed that by employing tapering magnets instead of parallel-sided magnets, the voltage of the electron beam detected by the target could be increased by 50%.

Figure 6 illustrates a device according to a second embodiment of the invention. As with the device of Figures 1 to 5, the device of Figure 6 comprises an electron gun 102 mounted at one end of an evacuated chamber in the form of a tube 104, the external features of which are shown in Figure 8. A target electrode 106 is mounted at the other end of the evacuated tube 104. The electron gun comprises a heating element 109 and a lanthanum hexaboride (LaB₆) emitter cathode 108. The electrons emitted upon heating the LaB₆ cathode are accelerated by the accelerating anode 110 in the manner described above in relation to Figure 1. Shaped magnets 112 of the type described above are attached to the outer surface of the evacuated tube 104.

The target electrode 106 is connected to a DC/AC converter 114 which may be of conventional construction or may be constructed as shown schematically in block diagram form in Figure 7. The DC/ AC converter converts the DC current drawn off from the target electrode 106 into 240V AC current.

The DC/ AC converter 114 has a power input 202 and a power output 204. Between the power input 202 and power output 204 are disposed a voltage divider 206, a sine wave generator 208, a converter array 210 comprising either a parallel array of photoconductors or a parallel array of transistors arranged in chopper circuits, a capacitor 212 and a transformer 214.

The voltage divider 206 comprises four high voltage low current resistors Rl, R2, R3 and R4 and is linked to the sine wave generator 208. The sine wave generator sends control signals to the converter array 210 so that the photoconductors or transistors are activated to superimpose a 50 Hz sinusoidal profile on the DC current passing through the converter array. The converter array 210 is connected in turn to a capacitor 212 which may be, for example a 0.1 μF, 2500V capacitor. The capacitor 212 blocks the passage of the DC component of the current but allows the passage of the sinusoidal (AC) component. The resulting 50 Hz AC current is directed through the transformer 214 where it is transformed from 1500V down to 240V.

A DC/AC converter of the type shown in Figure 7 can typically convert 0.33 A DC current at 1500V (500 Watts) into 1.97A AC current at 240V (473 Watts). The converter is provided with an output to enable the device to be connected to a range of power-consuming electrical and electronic devices.

Part of the electrical output from the converter 114 can also be directed via connection 116 to a 12V DC supply 118 which is used to power the electron gun and getter pump (see Figure 8). The DC supply 118 is linked via connection 120 to the heater 109 and via connection 121 to a Cockcroft Walton high voltage multiplier which transforms the 12V DC voltage up to voltages in excess of 1 kV for the accelerating electrode 110.

The external features of the evacuated tube 104 are illustrated in Figure 8. As shown in Figure 8, the evacuated tube 104 comprises a length of stainless steel tube which may be, for example, approximately 320 mm long, 38 mm in diameter and having a wall thickness of 1.68 mm.

At one end of the tube, a flange 130 is provided to form an input port for the electron gun (not shown). At the other end of the tube 104 is provided a flange 132 to which is bolted a four way flange connector manifold 134. Outlet flange 136 has attached thereto a two way isolation valve (not shown) which can be connected to a vacuum pump. Outlet flange 138 is attached to a getter pump (not shown) which assists in maintaining the vacuum in the tube. The getter pump can be any commercially available pump suitable for the purpose and one example of such a pump is the Sorb AC^® Appendage getter pump available from SAES Getters of Milan, Italy. Outlet flange 138 has the target electrode 106 connected thereto, the electrode being connected so as to extend approximately 10 mm into the tube 104 beyond the flange 132. By way of example, each of the flanges of the connector manifold 134 may have a 70 mm outside diameter, a 38 mm inside diameter and a depth of 8 mm. Each flange is provided with a plurality of fixing bolts (typically six per flange) for connecting the relevant item to the flange. The overall length of the flange connector manifold from the point of attachment at 132 to the outer surface of the outlet flange 138 may conveniently be of the order of 126 mm. In order to prevent loss of vacuum, sealing gaskets (typically of copper) are provided between confronting flange surfaces.

The device of the invention is constructed by connecting the electron gun, target electrode, getter pump and isolation valve to their respective flanges and then linking the isolation valve to a vacuum pump. Using the vacuum pump, the pressure inside the tube is initially reduced to a value of about 10^"9 Torr with heating to remove any gas molecules and water vapour within the tube. After a suitable period (for example 72 hours), during which the cathode of the electron gun may be heated to remove any gas molecules or water vapour adsorbed thereon, the isolation valve is closed and the vacuum pump removed. Thereafter, the vacuum is maintained at a level of about 10^" Torr by virtue of the integrity of the seals at the flange connections, with any minor leakages being compensated for by the getter pump.

The magnets (not shown in Figure 8), which are preferably ceramic magnets, are removably attached to the outer surface of the tube 104 by means of retaining clips (not shown).

The magnets shown in the Figures taper in a uniform manner along the length of the tube and hence the change in magnetic flux along the tube is essentially rectilinear. As an alternative, however, the edges of the magnets could be curved to give a similar non-rectilinear change in magnetic flux. As a further alternative, the edges of the magnets could be stepped, for example by using a range of part annular magnets.

The magnets shown in the Figures are typically each formed from a plurality (e.g. seven) of sections of magnetic alloy bonded together to form a smooth-sided trapezoidal shape. However, by bonding together a number of rectangular magnets of decreasing length and width, based on an ideal spiral, a tapering magnet with stepped edges can be realised. In a further variation, the magnets can be curved to form a spiral, e.g. an ideal spiral, wherein the magnets taper towards the centre of the spiral. Spiral shaped magnets are used when the evacuated chamber through which the stream of electrons passes is an annular chamber surrounding a central target electrode. In this embodiment, the stream of electrons is produced by an electron gun mounted tangentially in a radially outer wall of the evacuated annular chamber.

The device of the invention can be used as a power supply, for example as a portable auxiliary power supply in the event of mains failure. Although an electrical input is required initially in order to operate the electron gun and operate the getter pump, this can be provided by means of a battery. Once the device is running, a proportion of the power output can be diverted to the electron gun and getter pump to keep the device operating. Tests have established that the device can keep running under its own power for prolonged periods.

Equivalents

It will readily be apparent that numerous modifications and alterations may be made to the specific embodiments of the invention described above without departing from the principles underlying the invention. All such modifications and alterations are intended to be embraced by this application.

Claims

1. A device for producing and focussing an electron beam; the device comprising an evacuated chamber, a source of electrons, a target, means for creating an electric field to accelerate a stream of the electrons constituting the electron beam through the chamber towards the target, and means for providing a magnetic field crossing the electron beam; characterised in that the magnetic flux of the magnetic field progressively reduces in the direction of the target.

2. A device for producing and enhancing the energy of an electron beam; the device comprising an evacuated chamber, a source of electrons, a target, means for creating an electric field to accelerate a stream of the electrons constituting the electron beam through the evacuated chamber towards the target, and means for providing a magnetic field crossing the electron beam; characterised in that the flux of the magnetic field progressively reduces in the direction of the target and whereby the energy of the electron beam is enhanced as it passes though the magnetic field.

3. A device for producing and increasing the voltage of an electron beam; the device comprising an evacuated chamber, a source of electrons, a target, means for creating an electric field to accelerate a stream of the electrons constituting the electron beam through the evacuated chamber towards the target, and means for providing a magnetic field crossing the electron beam; characterised in that the flux of the magnetic field progressively reduces in the direction of the target, and whereby the voltage of the electron beam is increased as it passes though the magnetic field.

4. A power supply device; the power supply device comprising an evacuated chamber, means for producing a stream of electrons, a target connected to an output so that power can be drawn off the target, means for creating an electric field to accelerate the stream of electrons through the evacuated chamber towards the target, and means for providing a magnetic field crossing the electron beam; characterised in that the flux of the magnetic field progressively reduces in the direction of the target.

5. A device according to any one of the preceding claims wherein the magnetic field is substantially perpendicular to the electric field.

6. A device according to any one of the preceding claims wherein the magnetic flux reduces in a continuous manner.

7. A device according to any one of claims 1 to 5 wherein the magnetic flux reduces in a series of stepped reductions.

8. A device according to any one of the preceding claims wherein the rate of change in the magnetic flux is substantially constant.

9. A device according to any one of the preceding claims wherein the means for providing a magnetic field comprises an array of two or more shaped magnets wherein the shape of the magnets determines the reduction in magnetic flux in the direction of the target.

10. A device according to claim 9 wherein the shaped magnets are trapezoidal in plan and have a wider end and a narrower end, the narrow end being nearer the target.

11. A device according to claim 10 wherein the length of the shaped magnet is equal to the length of a path between two points P¹ and P² on a midline between two lines S¹ and S² each forming an ideal spiral, wherein the distance W¹ between the lines S¹ and S² at the outermost point P¹ on the ideal spiral corresponds to the width of the wider end of the trapezoidal magnet and the distance W between the lines S and S at the innermost point P on the ideal spiral corresponds to the width of the narrower end the trapezoidal magnet.

12. A device according to claim 9, wherein the shaped magnets are spiral shaped in plan and have a wider end and a narrower end, the narrow end being at the innermost point of the spiral, and wherein the evacuated chamber is an annular chamber which surrounds the target.

13. A device according to any one of claims 9 to 12 wherein the shaped magnets are of substantially uniform thickness along their length.

14. A device according to any one of claims 9 to 13 wherein the array of magnets comprises at least one pair of opposed magnets arranged about the stream of electrons.

15. A device according to claim 14 wherein there are at least two pairs of opposed magnets arranged about the stream of electrons.

16. A device according to any one of claims 9 to 15 wherein the magnets take the form of elongate strips that taper in the direction of the target.

17. A device according to any one of the preceding claims wherein the magnetic field strength provided by the magnets is at least 0.001 Tesla, more usually at least 0.01 Tesla, preferably at least 0.05 Tesla and most preferably at least 0.1 Tesla.

18. A device according to any one of the preceding claims wherein the magnets are ceramic magnets.

19. A device according to any one of the preceding claims wherein the evacuated chamber is evacuated to a pressure of 1 x 10^" Torr or less, more typically 5 x

10 Torr or less and most preferably 1 x 10^" Torr or less.

20. A device according to any one of the preceding claims wherein a getter pump is provided to assist maintenance of a vacuum within the evacuated chamber.

21. A device substantially as described herein with reference to the accompanying drawings.

22. A method of focussing an electron beam comprising the use of a device according to any one of the preceding claims.